Mine isn't a CX-5 but a CX-3 though I think the computer is pretty much the same.
My actual mpg is usually (about 95% of the time) within 0.5 MPG US, rest is of by 0.5-1.5.
Sometimes the actual mpg is better than the computer, sometimes worse.
Mind you I've had to convert from L/100km to MPG US...